Method of producing fuel dispersion for a fuel cell

ABSTRACT

A method of producing a fuel or concentrate thereof for a fuel cell. The method comprises providing a solution of at least one hydroxide ion providing compound in a liquid medium and combining and mixing one or more hydride compounds with this solution to provide a colloidal dispersion of the hydride compound(s) in the alkaline medium. This Abstract is neither intended to define the invention disclosed in this specification nor intended to limit the scope of the invention in any way.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. § 119(e) of U.S. provisional application No. 60/663,729, filed Mar. 22, 2005, the entire disclosure whereof is expressly incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for the production of a fuel or a concentrate thereof for use with a fuel cell.

2. Discussion of Background Information

Fuel cells are electrochemical power sources wherein electrocatalytic oxidation of a fuel (e.g., molecular hydrogen or methanol) at an anode and electrocatalytic reduction of an oxidant (often molecular oxygen) at a cathode take place simultaneously. Conventional fuels such as hydrogen and methanol pose several storage and transportation problems, in particular, for portable fuel cells (e.g., for use with portable electric and electronic devices such as laptops, cell phones, and the like).

Borohydride- (and other hydride) based fuels are of particular interest for portable fuel cells, due to their very high specific energy capacity (see, e.g., J. of Electrochem. Soc., 150, (3), A398-402, 2003). This type of fuels may be used either directly as the fuel or indirectly as a generator of hydrogen (which is oxidized at the anode), e.g., as part of a portable proton exchange membrane (PEM) fuel cell (see, e.g., US 20010045364 A1, US 20030207160 A1, US 20030207157 A1, US 20030099876 A1, and U.S. Pat. Nos. 6,554,877 B2 and 6,562,497 B2). The disclosures of all of the above documents are expressly incorporated herein by reference in their entireties.

The main oxidation reaction of a borohydride compound at the anode of a fuel cell can be represented as follows: BH₄ ⁻+8 OH⁻=BO₂ ⁻+6 H₂O+8e ⁻.

However, there also is a side reaction which leads to hydrogen evolution during the electrocatalytic oxidation: BH₄ ⁻+4 OH⁻=BO₂ ⁻+2 H₂O+2 H₂+4 e ⁻.

Moreover, during storage of a borohydride fuel, a spontaneous decomposition reaction usually takes place: BH₄ ⁻+2H₂O=BO₂ ⁻+4H₂.

The above decomposition reaction results not only in an undesirable decrease of the specific energy capacity of the borohydride fuel, but also causes storage and transportation problems due to, in particular, the generation of (highly flammable) hydrogen gas, which may also lead to a dangerous increase of the pressure inside a fuel reservoir.

One of the factors which has a strong influence on the decomposition rate of a borohydride fuel and other metal hydride fuels is the temperature. With increasing temperature, the decomposition rate increases exponentially. Also, the presence of catalytic impurities (salts of Ni, Fe, Co, Mg, Ca, etc.) may significantly affect (increase) the fuel decomposition rate.

Increasing the alkalinity of a borohydride fuel and, in general, of hydride containing liquids for use with fuel cells is an inexpensive and effective way of increasing the stability thereof. However, increasing the alkalinity of the fuel to a level which affords a desirable fuel stability for storage and transportation purposes will usually entail an impractical increase in the fuel viscosity (i.e., such that pumping of the fuel becomes difficult or even impossible), a decrease in the solubility of reaction products in the fuel and/or a drop in the specific energy capacity of the fuel. In particular, for practical purposes the optimum hydroxide concentration in a fuel will usually be in the range of from about 2 to about 6 mole/liter.

Co-pending U.S. patent application Ser. No. 10/757,849, filed Jan. 16, 2004 (U.S. Patent Application Publication 2005/0155279), the entire disclosure whereof is expressly incorporated by reference herein, discloses a storage-stable liquid concentrate for use with a fuel cell. The concentrate comprises at least one metal hydride compound in a liquid phase with a high alkalinity, typically with a hydroxide ion concentration of more than about 6 moles per liter. The present invention is directed to a process which is capable of affording corresponding concentrates and the like in the form of a substantially homogeneous and stable colloidal dispersion.

SUMMARY OF THE INVENTION

The present invention provides a method of producing a fuel or fuel concentrate for a fuel cell. The method comprises (a) providing a solution of one or more hydroxide ion providing compounds in a liquid medium; and (b) combining and mixing one or more hydride compounds (e.g., one or more metal hydride compounds) with the solution of (a) to provide a colloidal dispersion of the one or more hydride compounds in an alkaline medium.

In one aspect of the method, the solution of (a) may have a hydroxide ion concentration of at least about 0.2 mole per liter, e.g., at least about 0.5 mole per liter, at least about 1 mole per liter, at least about 2 moles per liter, at least about 3 moles per liter, at least about 4 moles per liter, at least about 6 moles per liter, or at least about 8 moles per liter. For example, the solution of (a) may have a hydroxide ion concentration of up to about 14 moles per liter, e.g., up to about 12 moles per liter, or up to about 10 moles per liter.

In another aspect, the solution of (a) may have a temperature of not higher than about 70° C., e.g., not higher than about 60° C., not higher than about 50° C., not higher than about 40° C., not higher than about 30° C., or not higher than about 25° C.

In yet another aspect of the method, the one or more hydride compounds may be employed in an amount which affords a total concentration of hydride compound(s) of at least about 2 moles per liter of colloidal dispersion (fuel or fuel concentrate), e.g., at least about 3 moles per liter, at least about 4 moles per liter, at least about 5 moles per liter, at least about 6 moles per liter, at least about 8 moles per liter, or at least about 10 moles per liter of colloidal dispersion.

In a still further aspect, at least two hydride compounds (e.g., two, three or four hydride compounds) may be employed. For example, the at least two hydride compounds may be added substantially successively, e.g., at least about 90% of a first hydride compound may be added before a second metal hydride compound is added to the solution of (a). In particular, when the solubility of a first metal hydride compound in the solution of (a) is higher than the solubility of a second metal hydride compound in the solution of (a) it is preferred according to the present invention to add at least a part (e.g., at least about 50%, at least about 70%, or at least about 90%) of the first metal hydride compound before the second metal hydride compound is added to the solution of (a). Regarding the use of metal hydride compounds of different solubility particular reference is made to co-pending U.S. patent application having the title “Fuel Composition for Fuel Cells” (Attorney Docket No. P28867), filed concurrently herewith, the entire disclosure whereof is expressly incorporated by reference herein.

In yet another aspect of the method, the one or more hydride compounds may be capable of undergoing an anodic oxidation in a liquid fuel cell and/or undergoing a decomposition with generation of hydrogen gas under conditions which promote a hydrolysis thereof.

In a still further aspect, the one or more hydride compounds may be selected from ammonium, alkali metal, and alkaline earth metal hydrides, borohydrides and aluminum hydrides such as, e.g., NaBH₄, KBH₄, LiBH₄, NH₄BH₄, Be(BH₄)₂, Ca(BH₄)₂, Mg(BH₄)₂, Zn(BH₄)₂, Al(BH₄)₃, polyborohydrides, (CH₃)₂NHBH₃, NaCNBH₃, LiH, NaH, KH, CaH₂, BeH₂, MgH₂, NaAlH₄, LiAlH₄ and KAlH₄. For example, the one or more hydride compounds may comprise one or more of NaBH₄, KBH₄, LiBH₄, NH₄BH₄ and a polyborohydride of formula MB₃H₈, M₂B₁₀H₁₀, MB₁₀H₁₃, M₂B₁₂H₁₂ or M₂B₂₀H₁₈ wherein M=Li, Na, K, NH₄, Be_(1/2), Ca_(1/2), Mg_(1/2), Zn_(1/2) or Al_(1/3).

In another aspect of the present method, the one or more hydride compounds may comprise at least one of NaBH₄ and KBH₄.

In another aspect of the method of the present invention, the one or more hydroxide ion providing compounds may be selected from hydroxides of alkali and alkaline earth metals, Zn and Al and from ammonium hydroxide such as, e.g., LiOH, NaOH, KOH, RbOH, CsOH, Ca(OH)₂, Mg(OH)₂, Ba(OH)₂, Zn(OH)₂, Al(OH)₃ and NH₄OH. For example, the one or more hydroxide ion providing compounds may comprise at least one of NaOH and KOH, preferably at least KOH.

In yet another (and preferred) aspect of the method of the present invention, the solution of (a) (and, thus the colloidal dispersion) may comprise water. For example, the colloidal dispersion which is afforded by the method of the present invention may comprise water and at least one water-soluble substance which is capable of stabilizing the colloidal dispersion. The at least one water-soluble substance may be present in the solution of (a) and/or may be added to the solution of (a) after and/or together with the one or more hydride compounds. (In this regard, if the solution of (a) comprises both water and at least a part of the at least one water-soluble substance, the hydroxide ion concentrations in the solution of (a) set forth above refer to the volume of water alone, i.e., without the at least one water-soluble substance.) The at least one water-soluble substance may be selected from, for example, one or more of aliphatic alcohols having up to about 6 carbon atoms and up to about 6 hydroxy groups, C₂₋₄ alkylene glycols, di(C₂₋₄ alkylene glycols), poly(C₂₋₄ alkylene glycols), mono-C₁₋₄-alkyl ethers of C₂₋₄ alkylene glycols, di(C₂₋₄ alkylene glycols) and poly(C₂₋₄ alkylene glycols), di-C₁₋₄-alkyl ethers of C₂₋₄ alkylene glycols, di(C₂₋₄ alkylene glycols) and poly(C₂₋₄ alkylene glycols), ethylene oxide/propylene oxide block copolymers, ethoxylated aliphatic polyols, propoxylated aliphatic polyols, ethoxylated and propoxylated aliphatic polyols, aliphatic ethers having up to about 6 carbon atoms, aliphatic ketones having up to about 6 carbon atoms, aliphatic aldehydes having up to about 6 carbon atoms, C₁₋₄-alkyl esters of C₁₋₄ alkanoic (aliphatic) acids and primary, secondary and tertiary aliphatic amines having a total of up to about 10 carbon atoms.

Non-limiting specific examples of the at least one water-soluble substance comprise methanol, ethanol, propanol, isopropanol, ethylene glycol, diethylene glycol, 1,2,4-butanetriol, trimethylolpropane, pentaerythritol, glycerol, sorbitol or any other sugar alcohol, acetone, methyl ethyl ketone, diethyl ketone, methyl acetate, ethyl acetate, dioxan, tetrahydrofuran, diglyme, triglyme, monoethanolamine, diethanolamine, triethanolamine, monopropanolamine, dipropanolamine and tripropanolamine. Alternatively or additionally, the at least one water-soluble substance may comprise a water-soluble polymer and/or a water-swellable polymer. For example, this polymer may have an initial particle size of from about 0.02 to about 50 microns.

In another aspect, the weight ratio of water and the at least one water-soluble substance may be from about 200:1 to about 2:1. For example, the weight ratio may be not higher than about 100:1, e.g., not higher than about 50:1, not higher than about 30:1, or not higher than about 20:1, and/or may be not lower than about 3:1, e.g., not lower than about 4:1, not lower than about 5:1, or not lower than about 10:1.

The method of the present invention may further comprise a treatment of the dispersion of (b) under subatmospheric pressure (vacuum) to remove gas bubbles which are entrapped therein, and/or may further comprise a heating (and preferably mixing) of the dispersion of (b), for example, to further homogenize the dispersion (and/or to promote a complete ion exchange). The heating temperature may, for example, be at least about 50° C., e.g., at least about 60° C., at least about 70° C., or at least about 80° C. Also, the heating (and mixing) may be carried out under subatmospheric pressure.

The present invention also provides a method of producing a fuel or fuel concentrate for a fuel cell, which method comprises (a) providing a solution of one or more hydroxide ion providing compounds in an aqueous medium, the solution having a concentration of hydroxide ions of at least about 0.2 mole per liter and being at a temperature of not higher than about 70° C.; and (b) adding one or more metal hydride compounds to the solution of (a) and mixing the one or metal hydride compounds with the solution to provide a colloidal dispersion of the one or more metal hydride compounds in an alkaline aqueous medium. The one or more hydride compounds are employed in an amount which results in a total concentration thereof of at least about 2 moles per liter of fuel or fuel concentrate.

In one aspect, the solution of (a) may have a hydroxide ion concentration of at least about 2, e.g., at least about 3, at least about 4, or at least about 5 moles per liter.

In another aspect, the one or more metal hydride compounds may be employed in a total amount of at least about 5 moles, e.g. at least about 6 moles, or at least about 7 moles per liter of fuel or fuel concentrate.

In a still further aspect, the one or more metal hydride compounds may comprise at least one of NaBH₄ and KBH₄ and/or the one or more hydroxide ion providing compounds may comprise at least one of NaOH and KOH.

In yet another aspect of the method, in addition to water, the aqueous medium of (a) may comprise at least one water-soluble substance selected from methanol, ethanol, propanol, isopropanol, ethylene glycol, diethylene glycol, glycerol, 1,2,4-butanetriol, trimethylolpropane, pentaerythritol, sorbitol or any other sugar alcohol, acetone, methyl ethyl ketone, diethyl ketone, methyl acetate, ethyl acetate, dioxan, tetrahydrofuran, diglyme, triglyme, monoethanolamine, diethanolamine, triethanolamine, monopropanolamine, dipropanolamine and tripropanolamine. For example, the weight ratio of water and the at least one water-soluble substance may be from about 200:1 to about 3:1. In this regard, the hydroxide ion concentrations in the solution of (a) set forth above are based on the volume of the solution without the at least one water-soluble substance.

The present invention also provides a method of producing a fuel or fuel concentrate for a fuel cell, which method comprises mixing one or more hydroxide ion providing compounds, one or more hydride compounds and one or more polar solvents in relative ratios which result in a colloidal dispersion of the one or more hydride compounds in an alkaline medium.

In one aspect of the method, the one or more hydroxide ion providing compounds may comprise one or more of an alkali or alkaline earth metal hydroxide, ammonium hydroxide, Zn(OH)₂ and Al(OH)₃ and/or the one or more hydride compounds may comprise NaBH₄, KBH₄, LiBH₄, NH₄BH₄, Be(BH₄)₂, Ca(BH₄)₂, Mg(BH₄)₂, Zn(BH₄)₂, Al(BH₄)₃ or a polyborohydride or a combination of two or more thereof, and the one or more polar solvents may comprise water and at least one water-soluble substance selected from aliphatic alcohols having up to about 6 carbon atoms and up to about 6 hydroxy groups, C₂₋₄ alkylene glycols, di(C₂₋₄ alkylene glycols), poly(C₂₋₄ alkylene glycols), mono-C₁₋₄-alkyl ethers of C₂₋₄ alkylene glycols, di(C₂₋₄ alkylene glycols) and poly(C₂₋₄ alkylene glycols), di-C₁₋₄-alkyl ethers of C₂₋₄ alkylene glycols, di(C₂₋₄ alkylene glycols) and poly(C₂₋₄ alkylene glycols), ethylene oxide/propylene oxide block copolymers, ethoxylated aliphatic polyols, propoxylated aliphatic polyols, ethoxylated and propoxylated aliphatic polyols, aliphatic ethers having up to about 6 carbon atoms, aliphatic ketones having up to about 6 carbon atoms, aliphatic aldehydes having up to about 6 carbon atoms, C₁₋₄-alkyl esters of C₁₋₄ alkanoic (aliphatic) acids and primary, secondary and tertiary aliphatic amines having a total of up to about 10 carbon atoms.

In another aspect of the method, the one or more hydroxide ion providing compounds may be employed in a total concentration of at least about 0.5 mole per liter per liter of fuel or fuel concentrate, e.g., in a total concentration of at least about 1 mole per liter, at least about 2 moles per liter, at least about 3 moles per liter, at least about 4 moles per liter, at least about 5 moles per liter, or at least about 6 moles per liter of fuel or fuel concentrate, and/or the one or more hydride compounds may be employed in a total concentration of at least about 2 moles per liter of fuel or fuel concentrate, e.g., in a total concentration of at least about 3 moles per liter, at least about 4 moles per liter, at least about 5 moles per liter, or at least about 6 moles per liter of fuel or fuel concentrate.

The present invention also provides a fuel or fuel concentrate which is obtainable by any of the methods of the present invention (including the various aspects thereof).

In one aspect, the fuel concentrate may have a Brookfield viscosity right after the preparation thereof of at least about 2,000 cP (as determined with a Brookfield viscosimeter at 22° C., 12 rpm, spindle S64, 20 sec), e.g., at least about 3,000 cP, at least about 4,000 cP, at least about 5,000 cP, or at least about 6,000 cP and/or the fuel concentrate may have a density of at least about 1.20 g/cm³, e.g., a density of at least about 1.25 g/cm³, or at least about 1.3 g/cm³.

The present invention also provides a method of producing a fuel for a fuel cell from a fuel concentrate. The method comprises a dilution with a polar liquid of a fuel concentrate which is obtainable by any of the methods of the present invention.

In one aspect of the method, both the fuel concentrate and the polar liquid may comprise water. For example, the fuel concentrate may comprise water and at least one water-soluble substance selected from one or more of aliphatic alcohols having up to about 6 carbon atoms and up to about 6 hydroxy groups, C₂₋₄ alkylene glycols, di(C₂₋₄ alkylene glycols), poly(C₂₋₄ alkylene glycols), mono-C₁₋₄-alkyl ethers of C₂₋₄ alkylene glycols, di(C₂₋₄ alkylene glycols) and poly(C₂₋₄ alkylene glycols), di-C₁₋₄-alkyl ethers of C₂₋₄ alkylene glycols, di(C₂₋₄ alkylene glycols) and poly(C₂₋₄ alkylene glycols), ethylene oxide/propylene oxide block copolymers, ethoxylated aliphatic polyols, propoxylated aliphatic polyols, ethoxylated and propoxylated aliphatic polyols, aliphatic ethers having up to about 6 carbon atoms, aliphatic ketones having up to about 6 carbon atoms, aliphatic aldehydes having up to about 6 carbon atoms, C₁₋₄-alkyl esters of C₁₋₄ alkanoic (aliphatic) acids and primary, secondary and tertiary aliphatic amines having up to about 10 carbon atoms.

In another aspect of the method, the polar liquid may comprise at least about 75% of water, e.g., at least about 80%, at least about 90%, at least about 95%, or at least about 99% by weight of water.

In yet another aspect of the method, the fuel concentrate may have a hydroxide ion concentration of at least about 0.5 moles per liter, e.g., at least about 1 mole, at least about 2 moles, at least about 3 moles, at least about 4 moles, at least about 5 moles, or at least about 6 moles per liter and/or the polar liquid may be added in an amount which results in a hydroxide ion concentration of the fuel of not more than about 90%, e.g., not more than about 80%, not more than about 70%, or not more than about 60% of the hydroxide ion concentration (in moles per liter) in the fuel concentrate. For example, the weight ratio of the fuel concentrate and the polar liquid may be from about 5:1 to about 1:5, e.g., not higher than about 4:1, or not higher than about 3:1, and not lower than about 1:4, e.g., not lower than about 1:3 or not lower than about 1:2.

In another aspect of the method, right after the production of the fuel concentrate, the concentrate may have a Brookfield viscosity of at least about 2,000 cP (as determined with a Brookfield viscosimeter at 22° C., 12 rpm, spindle S64, 20 sec), e.g., a Brookfield viscosity of at least about 3,000 cP, at least about 4,000 cP, at least about 5,000 cP, or at least about 6,000 cP, and the fuel may have a Brookfield viscosity of not higher than about 1,000 cP (as determined with a Brookfield viscosimeter at 23° C., 1 rpm, spindle S0, 1 min), e.g., a Brookfield viscosity of not higher than about 800 cP, not higher than about 600 cP, not higher than about 500 cP, or not higher than about 400 cP. In yet another aspect, the fuel concentrate may have a density of at least about 1.20 g/cm³, e.g., a density of at least about 1.25 g/cm³, or at least about 1.3 g/cm³, and/or the fuel may have a density of not higher than about 1.50 g/cm³, e.g. a density of not higher than about 1.40 g/cm³, not higher than about 1.30 g/cm³, not higher than about 1.20 g/cm³, or not higher than about 1.10 g/cm³.

The hydride compounds for use in the present invention preferably are compounds which can be oxidized as such at the anode of a fuel cell to provide electrons and/or can be used as a generator of molecular hydrogen which in turn is usable as a fuel in a fuel cell, e.g., by hydrolysis of the hydride compound. In other words, the fuel or concentrate thereof that is produced by the methods of the present invention may be suitable for a direct liquid fuel cell and/or for an indirect liquid fuel cell. It is to be understood that the term “hydride compound” as used in the present specification and the appended claims is used in a broad sense and encompasses, in particular, compounds which are “simple” hydrides, such as, e.g., NaH, KH, etc. as well as compounds which comprise a complex hydride ion such as, e.g., borohydride, aluminum hydride and the like. Non-limiting examples of metal hydride compounds for use in the present invention include hydrides, (poly)borohydrides, including cyanoborohydrides, and aluminum hydrides of alkali metals such as, e.g., Li, Na, K, Rb and Cs, and alkaline earth metals such as, e.g., Be, Mg, Ca, Sr and Ba, but also BH₃ complexes of organic amines such as, e.g., mono-, di- and trialkylamines. Corresponding specific compounds include, but are not limited to, NaBH₄, KBH₄, LiBH₄, NH₄BH₄, Be(BH₄)₂, Ca(BH₄)₂, Mg(BH₄)₂, Zn(BH₄)₂, Al(BH₄)₃, (CH₃)₂NHBH₃, NaCNBH₃, LiH, NaH, KH, CaH₂, BeH₂, MgH₂, NaAlH₄, LiAlH₄ and KAlH₄. Polyborohydrides may be used as well. Non-limiting examples of polyborohydrides are those of formulae MB₃H₈, M₂B₁₀H₁₀, MB₁₀H₁₃, M₂B₁₂H₁₂ and M₂B₂₀H₁₈ wherein M may be Li, Na, K, NH₄, Be_(1/2), Ca_(1/2), Mg_(1/2), Zn_(1/2) or Al_(1/3) (the fractions associated with Ca, Mg, Zn and Al take into account that these metals are bi- or trivalent). Further examples of polyborohydride compounds which are suitable for use in the present invention are disclosed in, e.g., U.S. Patent Application Publication 2005/0132640 A1, the entire disclosure whereof is incorporated by reference herein. Borohydrides and, in particular, metal borohydrides such as, e.g., NaBH₄ and KBH₄ are preferred for the purposes of the present invention.

The solution for use in step (a) of the method of the present invention preferably comprises one or more polar (protic and/or aprotic) solvent components. If the solution (a) is comprised of only one liquid component, i.e., if there is only one solvent component, this component will usually be polar and will preferably be water. If the solvent is a solvent mixture, i.e., comprises one or more (e.g., two, three, four, or even more) individual solvents, at least one of the components of the mixture should be polar. Preferably, all or at least substantially all of the solvent components are polar. If in addition to water one or more other polar solvents are present, the additional solvent(s) will preferably be capable of stabilizing an alkaline aqueous dispersion of the one or more hydride compounds. Other (water-soluble) substances such as water-soluble and/or water-swellable polymers (such as, e.g., crosslinked polyacrylic acid and salts thereof) may be used for this purpose as well. Solvents and solvent mixtures for use in the present invention preferably are liquid at room temperature. Non-limiting examples of suitable solvents include, besides water, mono- and polyhydric alcohols (e.g., methanol, ethanol, propanol, isopropanol, butanol, glycerol, trimethylolpropane and pentaerythritol) and mono-, di- and polyalkylene glycols (such as, e.g., ethylene glycol, diethylene glycol, propylene glycol, and dipropylene glycol), aliphatic esters of mono- and polycarboxylic acids (e.g., ethyl acetate, methyl acetate, ethyl formiate, and diethyloxalate), aliphatic ketones (such as, e.g., acetone, methyl ethyl ketone, and diethylketone), aliphatic aldehydes (such as, e.g., acetaldehyde, propionaldehyde and acetals thereof, (cyclo)aliphatic ethers (such as tetrahydrofuran, dioxane and partial or complete alkyl ethers of mono- and polyhydric alcohols and mono-, di- and polyalkylene glycols).

The hydroxide ion providing compound for use in the method of the present invention may be any compound which is capable of providing hydroxide ions in the solution of (a), e.g., by dissociation, decomposition, or by (in situ) reaction or interaction with any other compound that may be present in the solution. It will be understood that if the corresponding fuel is to be used in a direct liquid fuel cell, these compounds must not interfere to any significant extent with the operation of the fuel cell and, in particular, the electrochemical reactions that take place therein. Usually, the hydroxide ion providing compound will include at least one of an alkali metal hydroxide, an alkaline earth metal hydroxide, Zn(OH)₂, Al(OH)₃ and ammonium hydroxide. Non-limiting specific examples of suitable alkali and alkaline earth metal hydroxides include LiOH, NaOH, KOH, RbOH, CsOH, Ca(OH)₂, Mg(OH)₂ and Ba(OH)₂. The corresponding oxides, carbonates and bicarbonates are non-limiting examples of further compounds which may serve as hydroxide ion providing compounds for the purposes of the present invention. Often, NaOH and/or KOH will be employed. The amount of the hydroxide ion providing compound(s) is dependent on the desired hydroxide ion concentration in the solution of (a).

The hydroxide ion concentration of the solution of (a) that will provide the best results depends, inter alia, on the specific (metal) hydride compound(s), the solvent(s) and the amounts thereof, and the presence or absence of fuel additives and the like. Generally speaking, the hydroxide ion concentration in the solution of (a) will usually be at least about 0.2 mole per liter (although it may be lower), but will usually be not higher than about 14 moles per liter, preferably, not higher than about 12 moles per liter. Often, the hydroxide ion concentration will be at least about 0.4 mole per liter, e.g., at least about 0.6 mole per liter, at least about 0.8 mole per liter, at least about 1 mole per liter, at least about 2 moles per liter, e.g., at least about 3 moles per liter, at least about 4 moles per liter, or even at least about 6 moles per liter. If the solution of (a) contains both water and at least one water-soluble substance such as, e.g., a dispersion stabilizer, the above concentrations refer to the volume without the water-soluble substance(s).

The desirable concentration of the (metal) hydride compound(s) in the method of the present invention will frequently be somewhat related to the hydroxide ion concentration. In particular, because the dispersion product of the method of the present invention, in diluted form, will usually be intended for use as fuel/hydrogen generator for a fuel cell, the higher the hydroxide ion concentration in the dispersion and the lower the desired hydroxide ion concentration in the diluted dispersion (i.e., the fuel/hydrogen generator), the higher is the preferred concentration of the metal hydride compound(s) in the dispersion. In other words, after dilution of the dispersion to a desired hydroxide ion concentration, the resulting liquid should still contain a sufficient concentration of (metal) hydride compound(s) to be useful as fuel/hydrogen generator for a fuel cell. While the useful concentration of metal hydride compound(s) will depend, inter alia, on the fuel cell and the capacity thereof, as well as on many other factors, the colloidal dispersion which is obtained by the methods of the present invention will usually contain the metal hydride compound(s) in a concentration of at least about 2 moles per liter, preferably at least about 3 moles per liter, or at least about 4 moles per liter. In general, the (total) concentration of the metal hydride compound(s) in the colloidal dispersion will often be in the range of from about 2 moles per liter to about 12 moles per liter.

In order to increase the stability of the dispersion of the present invention, it is preferred for the dispersion to be substantially free of any substances which adversely affect the stability of the hydride compound(s) contained therein. For example, it may be desirable to have additives present in the fuel/hydrogen generator for the fuel cell, such as, e.g., one or more or plasticizers and detergents. The dispersion of the present invention is preferably free of such destabilizing additives or contains only minor quantities thereof (e.g., a total of less than about 0.1% by weight, even more preferred, less than about 0.01% by weight). It is also preferred for the dispersion to not contain any substances other than the hydride compound(s), the solvent or solvent components (and, other optional water-soluble substances which are capable of stabilizing an aqueous dispersion), respectively, and the hydroxide ion providing compound(s). If other substances are present, for example, stabilizers for the one or more hydride compounds, their total concentration preferably does not exceed about 5% by weight, and preferably does not exceed about 1% by weight. Unless otherwise indicated, the weight percentages given herein are based on the total weight of the dispersion.

Should it be desired for the final fuel/hydrogen generator to contain any substances which preferably are not present in the colloidal dispersion (or at least not in the desired concentrations), they may be added to the dispersion shortly before or during the dilution thereof. For example, all or a part of these desired substances may be added to the liquid (solvent) that is used for the dilution of the dispersion. It may, in particular, be advantageous to add one or more stabilizers for the hydride compound(s) to the diluent, because the diluted dispersion may no longer have a sufficiently high hydroxide ion concentration to satisfactorily stabilize the hydride compound(s) over extended periods of time. Non-limiting examples of suitable stabilizers (which can to some extent also be added during the production of the dispersion, for example, between steps (a) and (b) of the method of the present invention) include aromatic and aliphatic amines.

Regarding the production of the fuel or fuel concentrate of the present invention, while it is currently preferred to provide a solution of one or more hydroxide ion providing compounds (e.g., NaOH and/or KOH) in water and/or one or more of the polar solvents mentioned above and to add to this solution the one or more hydride compounds as such, whereafter the resultant mixture is subjected to agitation (e.g., by means of a (high speed) disperser and/or a planetary mixer and/or any other mixing apparatus) until a colloidal dispersion is obtained, one of ordinary skill in the art will appreciate that there are many other suitable ways of combining the components of the fuel (concentrate) of the present invention. For example, the components of the desired fuel (concentrate) may be combined all at once and then subjected to agitation. In another embodiment, a solution and/or dispersion of the one or more hydride compounds in a part of the liquid medium is added to a solution and/or dispersion of the one or more hydroxide ion providing compounds in the remainder of the liquid medium. In yet another embodiment, the one or more polar substances which may be employed to stabilize the colloidal dispersion may be a part of the liquid medium of (a) and/or may be added to the solution of (a) after and/or together with the one or more hydride compounds. These are but a few possible ways of combining the components for making the colloidal dispersion of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is further described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of exemplary embodiments of the present invention, wherein:

FIG. 1 shows a discharge curve in a direct borohydride—air fuel cell of the fuel prepared according to Example 1;

FIG. 2 shows a discharge curve in a direct borohydride—air fuel cell of the fuel prepared according to Example 2; and

FIG. 3 shows a discharge curve in a direct borohydride—air fuel cell of the fuel prepared according to Example 3.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The particulars shown herein are by way of example and for purposes of illustrative discussion of the embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the present invention. In this regard, no attempt is made to show details of the present invention in more detail than is necessary for the fundamental understanding of the present invention, the description making apparent to those skilled in the art how the several forms of the present invention may be embodied in practice.

EXAMPLE 1

A total of 35.94 g of granulated KOH (water content about 15% by weight) and 27.81 g of deionized water is placed in a plastic vessel and thoroughly mixed for about 2-3 minutes until a homogeneous clear solution is obtained. This solution is cooled to about 25° C., whereafter 4.19 g of ethylene glycol is added thereto. The resultant solution is transferred to a mortar and 29.06 g of potassium borohydride is added thereto. Thereafter the mortar is placed under vacuum (about 100 Torr) for about 30 minutes. The resultant mixture is then heated at about 70° C. for about 10 minutes and thereafter cooled to about 25° C. The resultant concentrate is mixed with deionized water (weight ratio 1:1.26) to produce a fuel for a direct fuel cell. The discharge curve of this fuel in a direct borohydride—air fuel cell (anode area=17 cm²; fuel volume 55 cc; discharge=0.6 V const.) is shown in FIG. 1.

EXAMPLE 2

A total of 30.86 g of granulated KOH (water content about 15% by weight), 16.65 g of NaOH and 19.81 g of deionized water is placed in a plastic vessel and thoroughly mixed for about 2-3 minutes until a homogeneous clear solution is obtained. This solution is cooled to about 25° C., whereafter 3.3 g of ethylene glycol and 1.2 g of sorbitol is added thereto. The resultant solution is transferred to a mortar and 29.58 g of sodium borohydride is added thereto. Thereafter the mortar is placed under vacuum (about 150 Torr) for about 30 minutes. The resultant mixture is then heated at about 60° C. for about 20 minutes and thereafter cooled to 25° C. The resultant concentrate is mixed with deionized water (weight ratio 1:1.33) to produce a fuel for a direct fuel cell. The discharge curve of this fuel in a direct borohydride—air fuel cell (anode area=17 cm²; fuel volume 55 cc; discharge=0.6 V const.) is shown in FIG. 2.

EXAMPLE 3

A total of 35.76 g of granulated KOH (water content about 15% by weight) and 32.2 g of deionized water is placed in a plastic vessel and thoroughly mixed for about 2-3 minutes until a homogeneous clear solution is obtained. This solution is cooled to about 25° C., whereafter 4.61 g of ethylene glycol is added thereto. The resultant solution is transferred to a mortar and 14.67 g of sodium borohydride is added thereto. Thereafter the mortar is placed under vacuum (about 100 Torr) for about 30 minutes. The resultant mixture is then heated at about 70° C. for about 10 minutes and thereafter cooled to about 25° C. The resultant concentrate is mixed with deionized water (weight ratio 1:0.88) to produce a fuel for a direct fuel cell. The discharge curve of this fuel in a direct borohydride—air fuel cell (anode area=17 cm²; fuel volume 55 cc; discharge=0.6 V const.) is shown in FIG. 3.

EXAMPLE 4

A total of 1747 g of deionized water is added to 1250 g of KOH (water content about 15% by weight). The resultant mixture is first stirred manually and then with a magnetic stirrer until a transparent solution is obtained. After the solution is cooled to room temperature (20-30° C.) it is transferred to the mixing can (volume about 4 L) of a combined high speed disperser and planetary mixer (PDM-2 from Ross, Hauppage, N.Y.). Following the addition of 210 g of ethylene glycol to the mixing can, the mixer is operated at 60 rpm (planetary mixer) and 600 rpm (high-speed disperser), respectively. After 5 minutes of mixing, 1495 g of KBH₄ is added to the mixture and mixing is continued for 40 minutes at 70 rpm (planetary mixer) and 600 rpm (high speed disperser), respectively, to afford a concentrate of paste-like consistency.

A fuel for a fuel cell is prepared from the concentrate by adding deionized water to the paste in a weight ratio paste:water of about 1:1.25. The resultant mixture is stirred at room temperature until a milky dispersion without lumps is obtained (with manual stirring usually after about 5-10 minutes).

EXAMPLE 5

According to a procedure similar to those described in Examples 1-4 above, a fuel concentrate is prepared from 31.72 parts by weight of KOH, 37.72 parts by weight of deionized water, 0.95 parts by weight of glycerol, 18.16 parts by weight of KBH₄, and 11.45 parts by weight of NaBH₄. The obtained concentrate paste has a density of 1.317 g/cm³ and a Brookfield viscosity (right after the preparation of the concentrate) of about 7,900 cP (22° C., 12 rpm, spindle S64, 20 sec). Dilution of the concentrate with deionized water in a weight ratio concentrate:water of about 1:1.25, affords a fuel having a density of 1.190 and a Brookfield viscosity of 272 cP (23° C., 1 rpm, spindle S0, 1 min).

EXAMPLE 6

According to a procedure similar to those described in Examples 14 above, a fuel concentrate is prepared from 21.05 parts by weight of KOH, 29.51 parts by weight of deionized water, 9.95 parts by weight of glycerol, 33.24 parts by weight of KBH₄, and 6.25 parts by weight of NaBH₄. The obtained concentrate paste has a density of 1.250 g/cm³ and a Brookfield viscosity (right after the preparation of the concentrate) of about 27,700 cP (22° C., 12 rpm, spindle S64, 20 sec). Dilution of the concentrate with deionized water in a weight ratio concentrate: water of about 1:1.25, affords a fuel having a density of 1.160 and a Brookfield viscosity of 420 cP (23° C., 1 rpm, spindle S0, 1 min).

EXAMPLE 7

According to a procedure similar to those described in Examples 14 above, a fuel concentrate is prepared from 32.01 parts by weight of KOH, 33.96 parts by weight of deionized water, 3.61 parts by weight of ethylene glycol, 18.74 parts by weight of KBH₄, and 11.60 parts by weight of NaBH₄. The obtained concentrate paste has a density of 1.310 g/cm³ and a Brookfield viscosity (right after the preparation of the concentrate) of about 6,600 cP (22° C., 12 rpm, spindle S64, 20 sec). Dilution of the concentrate with deionized water in a weight ratio concentrate:water of about 1:1.25, affords a fuel having a density of 1.190 and a Brookfield viscosity of 161 cP (23° C., 1 rpm, spindle S0, 1 min).

EXAMPLE 8

According to a procedure similar to those described in Examples 1-4 above, a fuel is prepared from 11.61 parts by weight of KOH, 66.61 parts by weight of deionized water, 18.34 parts by weight of KBH₄, and 3.45 parts by weight of NaBH₄. The obtained fuel has a density of 1.160 g/cm³.

EXAMPLE 9

According to a procedure similar to those described in Examples 1-4 above, a fuel concentrate is prepared from 33.10 parts by weight of KOH, 30.84 parts by weight of deionized water, 4.19 parts by weight of ethylene glycol, and 31.87 parts by weight of KBH₄. The obtained concentrate paste has a density of 1.350 g/cm³ and a Brookfield viscosity (right after the preparation thereof) of 361 cP (23° C., 1 rpm, spindle S0, 1 min).

The fuel (concentrate) prepared according to the process of the present invention can be used for both portable applications and for big power suppliers, e.g., for from about 0.1 W to about 100 kW.

It is noted that the foregoing examples have been provided merely for the purpose of explanation and are in no way to be construed as limiting of the present invention. While the present invention has been described with reference to exemplary embodiments, it is understood that the words which have been used herein are words of description and illustration, rather than words of limitation. Changes may be made, within the purview of the appended claims, as presently stated and as amended, without departing from the scope and spirit of the present invention in its aspects. Although the present invention has been described herein with reference to particular means, materials and embodiments, the present invention is not intended to be limited to the particulars disclosed herein; rather, the present invention extends to all functionally equivalent structures, methods and uses, such as are within the scope of the appended claims. 

1. A method of producing a fuel or fuel concentrate for a fuel cell, which method comprises: (a) providing a solution of one or more hydroxide ion providing compounds in a liquid medium; and (b) combining and mixing one or more hydride compounds with the solution of (a) to provide a colloidal dispersion of the one or more hydride compounds in an alkaline medium.
 2. The method of claim 1, wherein the solution of (a) has a hydroxide ion concentration of at least about 0.2 mole per liter.
 3. The method of claim 1, wherein the solution of (a) has a hydroxide ion concentration of at least about 2 moles per liter.
 4. The method of claim 2, wherein the solution of (a) has a hydroxide ion concentration of up to about 14 moles per liter.
 5. The method of claim 1, wherein the solution of (a) has a temperature of not higher than about 70° C.
 6. The method of claim 1, wherein the one or more hydride compounds are employed in an amount which affords a total concentration of at least about 2 moles per liter of colloidal dispersion.
 7. The method of claim 2, wherein the one or more hydride compounds are employed in an amount which affords a total concentration of at least about 5 moles per liter of colloidal dispersion.
 8. The method of claim 5, wherein the one or more hydride compounds are employed in an amount which affords a total concentration of at least about 7 moles per liter of colloidal dispersion.
 9. The method of claim 1, wherein at least two hydride compounds are employed.
 10. The method of claim 9, wherein the at least two hydride compounds are added substantially successively to the solution of (a).
 11. The method of claim 10, wherein a solubility of a first hydride compound of the at least two hydride compounds in the solution of (a) is higher than a solubility of a second hydride compound of the at least two hydride compounds and wherein at least a part of the first hydride compound is added to the solution of (a) before the second hydride compound is added thereto.
 12. The method of claim 1, wherein the one or more hydride compounds are capable of at least one of undergoing anodic oxidation in a liquid fuel cell and undergoing decomposition with generation of hydrogen gas under conditions which promote a hydrolysis thereof.
 13. The method of claim 1, wherein the one or more hydride compounds are selected from ammonium, alkali and alkaline earth metal hydrides, borohydrides and aluminum hydrides.
 14. The method of claim 1, wherein the one or more hydride compounds are selected from NaBH₄, KBH₄, LiBH₄, NH₄BH₄, Be(BH₄)₂, Ca(BH₄)₂, Mg(BH₄)₂, Zn(BH₄)₂, Al(BH₄)₃, a polyborohydride, (CH₃)₂NHBH₃, NaCNBH₃, LiH, NaH, KH, CaH₂, BeH₂, MgH₂, NaAlH₄, LiAlH₄ and KAlH₄.
 15. The method of claim 1, wherein the one or more hydride compounds comprise at least one of NaBH₄, KBH₄, LiBH₄, NH₄BH₄ and a polyborohydride of formula MB₃H₈, M₂B₁₀H₁₀, MB₁₀H₁₃, M₂B₁₂H₁₂ or M₂B₂₀H₁₈ wherein M=Li, Na, K, NH₄, Be_(1/2), Ca_(1/2), Mg_(1/2), Zn_(1/2) or Al_(1/3).
 16. The method of claim 15, wherein the one or more hydride compounds comprise at least one of NaBH₄ and KBH₄.
 17. The method of claim 1, wherein the one or more hydroxide ion providing compounds are selected from hydroxides of alkali and alkaline earth metals, Zn and Al and from ammonium hydroxide.
 18. The method of claim 17, wherein the one or more hydroxide ion providing compounds comprise at least one of LiOH, NaOH, KOH, RbOH, CsOH, Ca(OH)₂, Mg(OH)₂, Ba(OH)₂, Zn(OH)₂, Al(OH)₃ and NH₄OH.
 19. The method of claim 18, wherein the one or more hydroxide ion providing compounds comprise at least one of NaOH and KOH.
 20. The method of claim 1, wherein the solution of (a) comprises water.
 21. The method of claim 20, wherein the colloidal dispersion comprises water and at least one water-soluble substance which is capable of stabilizing the colloidal dispersion, the at least one water-soluble substance being at least one of present in the solution of (a) and being added to the solution of (a) at least one of after and together with the one or more hydride compounds.
 22. The method of claim 21, wherein the at least one water-soluble substance is selected from one or more of (cyclo)aliphatic alcohols having up to about 6 carbon atoms and up to about 6 hydroxy groups, C₂₋₄ alkylene glycols, di(C₂₋₄ alkylene glycols), poly(C₂₋₄ alkylene glycols), mono-C₁₋₄-alkyl ethers of C24 alkylene glycols, di(C₂₋₄ alkylene glycols) and poly(C₂₋₄ alkylene glycols), di-C₁₋₄-alkyl ethers of C₂₋₄ alkylene glycols, di(C₂₋₄ alkylene glycols) and poly(C₂₋₄ alkylene glycols), ethylene oxide/propylene oxide block copolymers, ethoxylated aliphatic polyols, propoxylated aliphatic polyols, ethoxylated and propoxylated aliphatic polyols, aliphatic ethers having up to about 6 carbon atoms, aliphatic ketones having up to about 6 carbon atoms, aliphatic aldehydes having up to about 6 carbon atoms, C₁₋₄-alkyl esters of C₁₋₄ alkanoic (aliphatic) acids and primary, secondary and tertiary aliphatic amines having a total of up to about 10 carbon atoms.
 23. The method of claim 21, wherein the at least one water-soluble substance is selected from one or more of methanol, ethanol, propanol, isopropanol, ethylene glycol, diethylene glycol, 1,2,4-butanetriol, trimethylolpropane, pentaerythritol, sorbitol, glycerol, acetone, methyl ethyl ketone, diethyl ketone, methyl acetate, ethyl acetate, dioxan, tetrahydrofuran, diglyme, triglyme, monoethanolamine, diethanolamine, triethanolamine, monopropanolamine, dipropanolamine and tripropanolamine.
 24. The method of claim 21, wherein the at least one water-soluble substance comprises at least one of a water-soluble polymer and a water-swellable polymer.
 25. The method of claim 24, wherein the polymer has an initial particle size of from about 0.02 to about 50 micron.
 26. The method of claim 21, wherein a weight ratio of water and the at least one water-soluble substance is from about 200:1 to about 2:1.
 27. A method of producing a fuel or fuel concentrate for a fuel cell, which method comprises: (a) providing a solution of one or more hydroxide ion providing compounds in an aqueous medium, the solution having a concentration of hydroxide ions of at least about 0.2 mole per liter and being at a temperature of not higher than about 70° C.; and (b) adding one or more metal hydride compounds to the solution of (a) and mixing the one or metal hydride compounds with the solution to provide a colloidal dispersion of the one or more metal hydride compounds in an alkaline aqueous medium; the one or more hydride compounds being employed in an amount which results in a total concentration thereof in the fuel or fuel concentrate of at least about 2 moles per liter.
 28. The method of claim 27, wherein the solution of (a) has a hydroxide ion concentration of at least about 2 moles per liter.
 29. The method of claim 28, wherein the one or more metal hydride compounds are employed in a total amount of at least about 5 moles per liter of fuel or fuel concentrate.
 30. The method of claim 29, wherein the one or more metal hydride compounds comprise at least one of NaBH₄ and KBH₄.
 31. The method of claim 30, wherein the one or more hydroxide ion providing compounds comprise at least one of NaOH and KOH.
 32. The method of claim 30, wherein the aqueous medium of (a) comprises water and at least one water-soluble substance selected from methanol, ethanol, propanol, isopropanol, ethylene glycol, diethylene glycol, 1,2,4-butanetriol, trimethylolpropane, glycerol, pentaerythritol, sorbitol, acetone, methyl ethyl ketone, diethyl ketone, methyl acetate, ethyl acetate, dioxane, tetrahydrofuran, diglyme, triglyme, monoethanolamine, diethanolamine, triethanolamine, monopropanolamine, dipropanolamine and tripropanolamine.
 33. The method of claim 32, wherein a weight ratio of water and the at least one water-soluble substance is from about 200:1 to about 3:1.
 34. A method of producing a fuel of fuel concentrate for a fuel cell, which method comprises mixing one or more hydroxide ion providing compounds, one or more hydride compounds and one or more polar solvents in relative ratios which result in a colloidal dispersion of the one or more hydride compounds in an alkaline medium.
 35. The method of claim 34, wherein the one or more hydroxide ion providing compounds comprise at least one of an alkali or alkaline earth metal hydroxide, ammonium hydroxide, Zn(OH)₂ and Al(OH)₃, the one or more hydride compounds comprise at least one of NaBH₄, KBH₄, LiBH₄, NH₄BH₄, Be(BH₄)₂, Ca(BH₄)₂, Mg(BH₄)₂, Zn(BH₄)₂, Al(BH₄)₃, and a polyborohydride, and the one or more polar solvents comprise water and at least one water-soluble substance selected from aliphatic alcohols having up to about 6 carbon atoms and up to about 6 hydroxy groups, C₂₋₄ alkylene glycols, di(C₂₋₄ alkylene glycols), poly(C₂₋₄ alkylene glycols), mono- and di-C₁₋₄-alkyl ethers of C₂₋₄ alkylene glycols, di(C₂₋₄ alkylene glycols) and poly(C₂₋₄ alkylene glycols), ethylene oxide/propylene oxide block copolymers, ethoxylated aliphatic polyols, propoxylated aliphatic polyols, ethoxylated and propoxylated aliphatic polyols, aliphatic ethers having up to about 6 carbon atoms, aliphatic ketones having up to about 6 carbon atoms, aliphatic aldehydes having up to about 6 carbon atoms, C₁₋₄-alkyl esters of C₁₋₄ alkanoic (aliphatic) acids and primary, secondary and tertiary aliphatic amines having a total of up to about 10 carbon atoms.
 36. The method of claim 35, wherein the one or more hydroxide ion providing compounds are employed in a total concentration of at least about 0.5 mole per liter of fuel or fuel concentrate and the one or more hydride compounds are employed in a total concentration of at least about 2 moles per liter of fuel or fuel concentrate.
 37. A fuel or fuel concentrate, obtainable by the method of claim
 34. 38. The fuel concentrate of claim 37, wherein the fuel concentrate has a Brookfield viscosity of at least about 2,000 cP.
 39. The fuel concentrate of claim 38, wherein the fuel concentrate has a density of at least about 1.20 g/cm³.
 40. A method of producing a fuel for a fuel cell from a fuel concentrate, wherein the method comprises diluting the fuel concentrate of claim 37 with a polar solvent.
 41. The method of claim 40, wherein both the fuel concentrate and the polar solvent comprise water.
 42. The method of claim 41, wherein the fuel concentrate comprises water and at least one water-soluble substance selected from one or more of aliphatic alcohols having up to about 6 carbon atoms and up to about 6 hydroxy groups, C₂₋₄ alkylene glycols, di(C₂₋₄ alkylene glycols), poly(C₂₋₄ alkylene glycols), mono- and di-C₁₋₄-alkyl ethers of C₂₋₄ alkylene glycols, di(C₂₋₄ alkylene glycols) and poly(C₂₋₄ alkylene glycols), ethylene oxide/propylene oxide block copolymers, ethoxylated aliphatic polyols, propoxylated aliphatic polyols, ethoxylated and propoxylated aliphatic polyols, aliphatic ethers having up to about 6 carbon atoms, aliphatic ketones having up to about 6 carbon atoms, aliphatic aldehydes having up to about 6 carbon atoms, C₁₋₄-alkyl esters of C₁₋₄ alkanoic (aliphatic) acids and primary, secondary and tertiary aliphatic amines having up to about 10 carbon atoms.
 43. The method of claim 42, wherein the polar solvent comprises at least about 75% by weight of water.
 44. The method of claim 41, wherein the fuel concentrate has a hydroxide ion concentration of at least about 0.5 mole per liter and the polar solvent is added in an amount which results in a hydroxide ion concentration of the fuel of not more than about 90% of the hydroxide ion concentration of the fuel concentrate.
 45. The method of claim 40, wherein a weight ratio of the fuel concentrate and the polar solvent is from about 5:1 to about 1:5.
 46. The method of claim 40, wherein the fuel concentrate has a Brookfield viscosity of at least about 2,000 cP and the fuel has a Brookfield viscosity of not higher than about 1,000 cP.
 47. The method of claim 40, wherein the fuel concentrate has a density of at least about 1.20 g/cm³.
 48. The method of claim 40, wherein the fuel has a density of not higher than about 1.50 g/cm³. 